The present invention relates to a color image forming apparatus, and more particularly relates to an electrostatic type color image forming apparatus such as a laser beam printer (LBP) and a digital color copier for which laser beams are utilized.
With the progress of computer graphics, there is a strong demand for very accurate and fine color images to be printed through highly advanced image processing. In the field of color copiers, clear color images are required in which the gradation is accurately reproduced and further accurate halftone images are obtained in accordance with the characteristics of the object to be copied.
In the field in which images of high quality are required, image formation is generally carried out in the following manner: a photoreceptor is scanned by a laser beam to form an electrostatic latent image; the electrostatic latent image is developed with color toners of Y (yellow), M (magenta), C (cyan) and BK (black), wherein the developed color toner images are put on each other; and the formed image is transferred onto a transfer paper by one operation. In order to satisfy the aforementioned various requirements, various systems are adopted for image processing.
As office automation apparatus have come into wide use, there is a demand for a printer of high image quality even in a small-scale office. In this case, improvements in the performance of apparatus are incompatible with the cost and size. Therefore, it is very difficult to satisfy all the requirements for the apparatus.
For example, in the case of image formation in which image writing processing is carried out at a standard setting, the cost can be reduced, however, the apparatus can not be controlled in accordance with the characteristics of various images, so that the following problems are caused: the hue of the entire image is insufficient; the hue is changed at the edge of an image; the contour of a sharp image or a diagonal line can not be reproduced accurately; the amount of moire is increased in the reproduction of a halftone dot image; and resolution is lacking when small characters of less than 5.5 point, so that the image quality is deteriorated. As a result of the foregoing, the aforementioned image forming system is not suited for obtaining images of high quality.
When it is attempted to improve image quality by means of image processing, various mechanisms are required in order to quickly change the processing conditions such as a highly developed signal processing circuit and control mechanism. As a result, the cost is raised.
In order to realize a multicolor printer of high image quality at low cost, the inventor investigated a technique in which the dot size and dot position in one pixel are dynamically changed with regard to the reference position. According to the result of the investigation conducted by the inventor, in the case of a monocolor image, the aforementioned dynamic processing (referred to as smoothing processing, hereinafter) to change the dot size and dot position is very effective to improve resolution and conduct smoothing on an image.
However, in the case of color image formation in which 2 color toner layers of Y, M, C and the like are put on each other, the change of dot size and dot position obstructs the alignment of the first and second color images. Therefore, the quality of the obtained color image is lower than that of a mono color image.
In order to realize a multicolor printer of high image quality at low cost, the inventor made an investigation. As a result of the investigation, it is apparent that, compared with monocolor image formation, color image formation has a problem of color alignment, so that it is difficult to improve image quality.